As is well known in the surgical instrument technology, trepans or cranial drills or perforators that serve to drill a hole in the skull must stop instantly upon reaching a certain depth. It is abundantly important that the drilling does not go beyond the depth of the skull and penetrate through the dura underlying the bone structure and/or the brain underlying the dura. There are a plethora of prior art patents that disclose cranial drill instruments that are intended to provide this function and for the most part these instruments utilize a concentric type of perforator or cranial drill that include an inner bit (front drill head assembly) and an outer bit (trailing outer drill). The inner bit senses the load occasioned by the operator applying pressure to the drill which is forced against the bone structure and when the inner bit no longer feels the resistive force the drilling stops by virtue of a clutching type of mechanism that responds to the axial position of the inner drill bit. Patents that exemplify this type of drill or perforator are U.S. Pat. No. 4,600,006 granted to Baker on Jul. 15, 1986 entitled "Cranial Perforator", U.S. Pat. No. 5,007,911 granted to Baker on Apr. 16, 1991, entitled "Drill Head Assembly For Cranial Perforators", U.S. Pat. No. 4,884,571 granted to Baker on Dec. 5, 1989, entitled "Cranial Perforator With Reentrant Cutting Segment", U.S. Pat. No. 4,699,550 granted to Baker on Oct. 13, 1987 entitled "Cranial Perforator", U.S. Pat. No. 5,330,480 granted to Meloul et al on Jul. 19, 1994 entitled "Surgical Drill", U.S. Pat. No. 4,951,690 granted to Baker on Aug. 28, 1990 entitled "Method Of Drilling Through A Bone Structure", U.S. Pat. No. 4,803,982 granted to Baker on Feb. 14, 1989 entitled "Cranial Perforator", U.S. Pat. No. 4,362,161 granted to Reimels et al on Dec. 7, 1982 entitled "Cranial Drill", U.S. Pat. No. 4,456,010 granted to Reimels et al on Jun. 26, 1984 entitled "Cranial Drill", and U.S. Pat. No. 4,830,001 granted to Walus on May 16, 1989 entitled "Assembly Sleeve For Cranial Drill".
In these types of instruments noted in the above-referred to patents, the drill bit applies motion to the clutch in order to bring the drill to an eventual stop. This occurs when the inner bit no longer feels the resistive force. Typically the instrument utilizes a pin and slot-type spring biased clutch that links the drill body and drill member together. Then the drill is placed against the bone structure with a force sufficient to overcome the spring bias, the clutch will engage and the driver, drill member and drill body will rotate together. The counterbore on the drill body provides a support for the drill mechanism so that the drill member may release when it penetrates the bone structure without having the remainder of the drill move in the direction toward the cranial cavity. Obviously, the clutch mechanism must displace a certain axial distance before the drill comes to a complete stop. In certain instances, the depth of penetration of the drill owing to this axial distance may present a problem.
An important aspect of this invention is that it eliminates the clutch mechanism of the type that is commercially available and shown in the prior art patents listed above. This invention provides means inherent in the perforator that immediately stops the penetration when the drill bit no longer feels a resistive force. In the perforator of the present invention the drilling forms an annular groove cut into the bone structure such that a portion of the bone in the form of a core or plug remains intact whereby the plug can be reused to partially fill that same hole that was previously drilled. This, obviously, has medical benefits in the closure of the hole in the bone structure and helps in the recovery process of the patient as well.
In the present invention the perforator or drill bit will automatically stop cutting (rotation is stopped when the operator deactivates the drill motor) when the load on the drill bit or perforator is removed. The perforator is cylindrical in shape defining a torroidal bottom edge much like the shape of a cup. The cutting edge is configured within the circumference of the bottom edge and lies in the circumference and facing the next adjacent circumferential portion which serves as a shield. The shield is spring loaded to deflect axially upwardly when forced against a resistive force exposing the edge of the cutting edge to define the cutting position. When the resistive force is eliminated the shield automatically returns to the original position to shield the cutting edge into the non-cutting position.
The invention contemplates using a stop pin assembly that rotates with the drill bit of the perforator and displaces the same distance of the drill bit so that the stop pins remain a constant distance away from the surface of the exterior of the bone structure being drilled during the drilling operation. In other words, if the prongs are set to be spaced say, 0.3 millimeter (mil), the gap between the end of the stop pins and the bone structure will remain this exact distance from the start of the drilling operation until the end of the drilling operation and only additional force exerted on the perforator by the operator will cause the perforator to move axially. The additional axial movement of the perforator will cause the stop pins to move until it penetrates the 0.3 mil gap and come into contact with the bone structure. This will limit penetration of the drill bit of the perforator beyond the cranial drilled hole which is well within safe distance from contacting the dura. This safety feature will assure that the perforator will not be displaced more than the 0.3 mil setting once the resistive force is removed from the drill bit. Obviously, the stop pins support assembly is merely a carry-on to assure that the surgeon doesn't inadvertently injure the dura in a cranial operation by exerting more force than is required to accomplish the drilling. In other types of operations where the drilling is not as exacting and penetration threshold need not be considered, the drill of this invention would have utility without the safety features. Obviously the gap setting of the stop pins can be predetermined as will be explained in connection with the description of the invention that is to follow. As is apparent from the foregoing, the stop pin feature and the ease of drilling with the present invention overcomes some of the resistance to use perforators as expressed by surgeons who are not confident of being able to stop the penetration in time to avoid injury to the dura when using the prior art perforators.
While the invention can be made into a reusable instrument, it would probably have more appeal in certain areas of the intended use of the instrument to be made as a throw-a-way instrument. In either case, the invention is characterized as being relatively inexpensive to make and easy to operate. As will be detailed in the description to follow two embodiments will be disclosed. The first embodiment represents the second design that included a minimum number of components. The second embodiment represents the first design which included many more components than is disclosed in the first embodiment. It will be appreciated by one skilled in the art that either of the embodiments have utility and that each perform the same functions and illustrate that a number of embodiments can be made without departing from the scope of this invention.
In actual testing of the two embodiments of the present invention it was observed by the operators that in drilling the bone structure of a skull, less force was needed by the operator than was required in comparison to the force required when a commercially available perforator of the type described in some of the above reference patents. It was also observed that when the drill bit stopped its cutting the stoppage was so exacting that an extremely thin partial sliver of the bone structure at the bottom of the plug that extending the width of the annular channel drilled by the perforator remained attached to the bone plug. This meant that the automatic stoppage of the cutting by the drill bit occurred before the bottom end of the perforator went beyond the bottom surface of the bone structure. To eliminate the sliver from occurring the cutting edge of the drill bit can be modified to include a contour on the side face of the cutting edge. The side face of the drill bit is the outer circumferential surface.
In certain applications, it is desirable to stop rotation of the drill bit at some point in the drilling operation. This is particularly the circumstance where the bone structure is such that the top surface of the bone is configured differently than the bottom surface and the topography is drastically different. This obviously changes the thickness of the bone in these locations resulting in large transitions of the thickness of the bone structure. In such circumstances, a portion of the drill bit will evidence a void while the remaining portion of the drill bit is in contact with bone structure. Under such circumstances, it may be desirable in ceasing the drilling operation. This invention contemplates utilizing a clutching mechanism that senses the void and declutches the drill bit from the drill motor to stop rotation of the drill bit and hence, cease the drilling operation.